Imaging modalities such as nuclear magnetic resonance imaging and ultrasound use various types of waves to interact with a sample of tissue such as mammalian tissue. In the case of ultrasound, mechanical waves in the form of acoustic waves or sound waves are used. Sound waves can propagate as longitudinal waves and shear waves in tissue. The characteristics and use of these wave types have different properties and applications relative to various imaging modalities.
In the case of longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Because compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids as well as solids because the energy travels through the atomic structure by a series of compressions and expansion (rarefaction) movements.
In a transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation and, therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves. The velocity of shear waves through a material is approximately half that of the longitudinal waves. In addition, in the ultrasound context, the angle in which an ultrasonic wave enters a material determines whether longitudinal, shear, or both types of waves are produced. Shear waves have an inherent polarization direction depending on how they are generated.
These properties of shear waves facilitate their use for various elasticity-related data collection scans with respect to a sample of interest that includes a suitable propagation media. The human body and other tissues satisfy this criterion. To detect the elasticity of a sample of interest such as tissue, various methods exist. However, in light of the various properties and imaging applications of shear waves, in addition to adding to such methods, a need exists to find ways to tailor the shear waves themselves and improve the associated imaging systems that use them.